The invention relates generally to systems for anti-islanding protection of a synchronous machine based distributed generator from a feeder having been disconnected from an electrical grid.
The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an electrical grid. The electrical grid includes independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed generator (DG). A typical DG includes some type of power conditioner or converter, such as an inverter for example, that feeds power to the feeder system of the grid. Exemplary DGs include but are not limited to energy storage devices (such as batteries or flywheels, for example), photovoltaics, micro-turbines, fuel cells, engine-generator sets, and wind-turbine-generator sets. A conventional feeder system typically includes distribution lines that provide power from the grid or DG to a customer load via electrical disconnects and distribution transformers. Even with the presence of a DG connected to the grid, the utility company is still the main source of power and in many cases controls the system voltage and frequency within nominal values.
Under certain conditions, the utility power source may be disconnected from the grid and feeder system, leaving the DG directly tied to the load or disjointed grid branch, which is referred to as islanding. The isolated section of the grid being powered by the DG is referred to as an island. Unintentional islanding results in a situation where the voltages and frequencies on the disjointed grid branch are outside of the direct control of the utility company because that branch is primarily energized by one or more DGs. Accordingly, monitoring and disconnect schemes, referred to as anti-islanding schemes, are used to timely disconnect a DG from the feeder in the event that grid power from a utility company has been disconnected from the feeder.
Anti-islanding schemes presently used or proposed include passive schemes and active schemes. Passive schemes are based on local monitoring of the grid signals, such as under or over voltage, under or over frequency, rate of change of frequency, phase jump, or system harmonics, for example. Active schemes are based on active signal injection with monitoring of the resulting grid signals, such as impedance measurement for example, or active signal injection with active controls, such as active frequency shifting or active voltage shifting for example. With passive schemes, close power matching between the DG output and the total load may result in a sustained island due to the voltage and frequency holding within nominal ranges. With active schemes, some distortion may occur in the output current waveform, thereby resulting in a tradeoff between islanding detection time and waveform distortion, with faster detection typically resulting in higher total harmonic distortion (THD). Accordingly, there is a need in the art for an anti-islanding arrangement that reduces these drawbacks.